1. Field of the Invention
The present invention relates to a method and an apparatus for aligning a first object an a second object, and also to a method and an apparatus for providing a desired gap between a first and a second object. More particularly, the invention relates to a method and an apparatus for aligning a mask and a wafer to transfer a circuit pattern image onto the wafer, and also to a method and an apparatus for providing a desired gap between a mask and a wafer to transfer a circuit pattern image onto the wafer.
2. Description of the Related Art
In manufacturing a semiconductor device such as a VLSI, an exposure apparatus transfers the circuit pattern of the device onto a semiconductor wafer. More specifically, the apparatus applies X-rays to a mask having the circuit pattern, and those X-rays passing through the mask are then applied to the wafer, thereby transferring the image of the pattern onto the wafer. In order to transfer the circuit pattern onto the wafer, it is necessary to align the mask and the wafer correctly with respect to each other, and also to provide a desired gap between the mask and the wafer.
The inventors hereof have proposed a method and an apparatus for aligning objects which can be applied to the transfer of a circuit pattern onto a semiconductor wafer, in U.S. Pat. Application No. 07/060,663 and also in the corresponding West German Patent Application P 37 19 539.5. In the method claimed in these patent applications, two diffraction gratings are formed on the mask and the wafer, respectively. A laser beam is applied to the diffraction grating on the mask, and is diffracted as it passes through the grating. The diffracted beam is applied to the diffraction grating on the wafer, and is diffracted as it passes through this grating. The laser beam, twice diffracted, is reflected from the wafer and applied back to the grating of the mask through the grating of the wafer. Hence, the beam is diffracted for the third time by the grating of the mask, thus forming a thrice diffracted light beam. The intensity of this light beam, which represents the displacement of the mask with respect to the wafer, is detected by a photosensor. The photosensor generates an electrical signal. In accordance with the signal, an aligning mechanism moves the mask, thereby correctly aligning the mask with respect to the wafer.
The reflectivity of the diffraction grating formed on the wafer may alter, and the permeability of the diffraction grating formed on the mask may also change. In particular, the reflectivity of the grating formed on the wafer changes ten and odd times during the process of manufacturing a VLSI. This is because the wafer is exposed to light ten and odd times during the manufacture of the VLSI, and resist is coated on the wafer each time the wafer is exposed to the light.
Whenever the reflectivity of the grating formed on the wafer or the permeability of the grating formed on the mask alters, the intensity (i.e., amplitude) of the thrice diffracted light beam changes. Consequently, the signal output by the photosensor has an insufficient S/N ratio. Due to the small S/N ratio of the signal, the aligning mechanism can no longer align the mask with respect to the wafer, with the required precision.
The gap between the mask and the wafer is adjusted to a desired value, also in accordance with the intensity of the thrice diffracted light beam. Therefore, the gap cannot be adjusted as accurately as desired, whenever the reflectivity of the grating formed on the wafer or the permeability of the grating formed on the mask varies, inevitably changing the intensity of the thrice diffracted light beam, and ultimately causing the photosensor to generate an electrical signal having an insufficient S/N ratio.